A civilian U.S. satellite has captured images of a huge underground runway being built near Wonsan, North Korea.

Voice of America (VOA) says satellite photo provider Google Earth confirmed through aerial photos that an airstrip is being built southwest of Wonsan. It's estimated to be one-point-eight kilometers long and 30 meters wide.

The U.S. government-funded broadcaster says the cement runway runs diagonally from northeast to southwest, adding that its entrance is connected to a mountain. VOA said tunnel construction appeared to be ongoing, with soil and rocks piled alongside the runway and some 20 buildings standing nearby.

Citing former North Korean Air Force Captain Pak Myong-ho, who defected to the South in May 2006, the report said the facility is most likely a military airstrip.